A configuration for controlling driving of an AC electric motor by converting DC voltage to AC voltage using a converter has been generally used. In such a configuration, generally, motor current is controlled in accordance with pulse width modulation (PWM) based on vector control. Further, configurations for controlling AC electric motor by switching between a rectangular wave voltage phase control mode in which drive control is attained by applying a rectangular wave voltage to the AC electric motor and a PWM current control mode based on PWM control have been known from Japanese Patent Laying-Open Nos. 2005-218299, 2001-78495, 2002-223590 and 2005-45880.
In the drive controllers for AC electric motors disclosed in these references, basically, mode switching determination is executed between the PWM current control mode (hereinafter also simply referred to as PWM control mode) and the rectangular wave voltage phase control mode (hereinafter also simply referred to as rectangular wave voltage control mode) based on the state of AC electric motor and, more specifically, on the voltage amplitude and current phase.
In the drive controllers for the AC electric motors mentioned above, the rectangular wave voltage control mode is used relatively in the high speed range of the AC electric motor. Therefore, in order to ensure control response in the rectangular wave voltage control mode, it is necessary to execute control calculation in a short period. As a result, a situation may occur in which control period of the rectangular wave voltage control mode becomes shorter than period of control mode switching determination executed as a part of a main loop for overall control of the AC electric motor. If such a situation occurs, control operation could possibly be unstable particularly at the transition from the rectangular wave voltage control mode to the PWM control mode.